This invention relates to an air-fuel ratio feedback control system and method for internal combustion engines which compensates for a response delay of air-fuel ratio detection at the engine exhaust side relative to changes in fuel injection at the engine intake side.
In recent electronically-controlled internal combustion engines, the amount of fuel supplied or injected into the engine is feedback-controlled in response to outputs of an oxygen sensor or an air-fuel ratio sensor disposed in the engine exhaust pipe so that the air-fuel ratio is regulated to a target air-fuel ratio to increase the exhaust purification efficiency of a three-way catalyst disposed in the engine exhaust pipe. The accuracy in the feedback control is rather lessened during transient engine conditions such as the engine acceleration or deceleration in which the amount of fuel rapidly changes, if the feedback control is effected during such a period in the similar manner as in the normal engine operation conditions.
It is to be noted that a part of fuel injected from fuel injectors sticks to intake port inner walls or intake valves of an engine and thereafter gradually evaporates and enters into cylinders causing a fuel supply transport time delay. Further, other time delays are caused, because the exhaust gas travels from the engine to the position of an oxygen sensor and the oxygen sensor has response time to detect the oxygen concentration in the exhaust gas.
Thus, the feedback control system has the fuel supply transport time delay and the air-fuel ratio detection time delay, from changes in the fuel injection amount at the engine intake side to the air-fuel ratio detection at the engine exhaust side. Those delays may result in a time period which corresponds to four to six rotations of the engine crankshaft.
The above delays cause a time delay in the feedback correction amount calculated based on the detected air-fuel ratio. The feedback correction amount changes with a time delay, if the amount of fuel to be injected is calculated in response to the feedback correction amount calculated based on the past fuel injection. That is, the feedback control does not follow quickly and lessens the accuracy in the feedback control, particularly when the fuel injection amount changes rapidly due to changes in the engine operation conditions.
It is therefore proposed in U.S. Pat. No. 4,586,478 (JP-A-58-27847) to stop the feedback control during the engine transient conditions. It is further proposed in JP-B1-7-26572 to stop or minimize the feedback control for a predetermined period when the engine transient condition is a rapid acceleration or deceleration, while effecting the normal feedback control when the engine transient condition is a slow acceleration or deceleration. The rapid acceleration or deceleration is defined as an acceleration or deceleration occurring immediately after the previous engine acceleration or deceleration, and the slow acceleration or deceleration is defined as an acceleration or deceleration occurring from the normal engine conditions. However, those proposed feedback controls are not satisfactory to compensate for the air-fuel ratio detection time delay.